The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, a thin layer of air develops between the slider and the rotating disk. When the slider rides on this air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos ⊖, where ⊖ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
Traditionally, GMR sensors have been constructed as current in plane (CIP) GMR sensors, wherein current flows through the sensor from one side to the other in a direction parallel with the planes of the layers making up the sensor. More recently, increased attention has been focused on current perpendicular to plane (CPP) GMR sensors. As its name suggests, in a CPP sensor, current flows through the sensor from top to bottom in a direction perpendicular to the planes of the layers making up the sensor.
Another type of magenetoresistive sensor is a tunnel junction sensor (TMR) or tunnel valve. A tunnel valve includes a pinned layer and a free layer, similar to a GMR sensor. However, instead of having a non-magnetic electrically conductive spacer layer between the free and pinned layers, a tunnel valve has a thin dielectric, non-magnetic barrier layer, which can be constructed of for example alumina Al2O3. A tunnel valve operates based on the spin dependent scattering of electrons through the thin barrier layer. When the magnetic moments of the free and the pinned layer are aligned parallel with one another, electrons much more readily pass through the barrier layer than when they are the moments are antiparallel. Therefore, current travels through a tunnel valve in a direction perpendicular to the plane of the layers making up the sensor, similar to a current perpendicular to plane (CPP) GMR.
With reference to FIG. 1A, magnetoresitive sensors have traditionally been constructed by a method that includes first depositing the sensor layers 102 as full film layers, on a substrate 104, which can be for example an alumina gap layer, or in the case of a CPP GMR or tunnel valve could be an electrically conductive magnetic material such as NiFe. Then, a full film layer of material that is resistant to chemical mechanical polishing (CMP stop) 106 is deposited. A layer 108 of antireflective coating (ARC) material, such as Duramide, that is resistant to removal by ion milling is then deposited over the CMP resistant material 106. A mask 110 that includes one or more layers of a photoreactive material such as photoresist is then formed to cover the area where the sensor is desired and exposing other portions. A reactive etching process is then used to remove the ARC and CMP resistant materials.
With reference now to FIG. 1B, a reactive ion etch 112 is then performed to remove portions of the ion mill resistant layer 108 and CMP stop 106 that are not covered by the photoresist mask 110. This process is known in the industry as transferring the image of the photoresist mask 110 onto the underlying mask layers 106, 108. The RIE process used to transfer the image of the photoresist mask onto the underlying layers 106, 108 is chosen to be a RIE process that can readily remove the materials making up the underlying layers 106, 108.
With reference to FIG. 1B, it can be seen that, since the RIE process used to perform the image transfer preferentially removes the layer 108 at a faster rate than it removes the photoresist layer 110, a bulbous or mushroom shape forms on the mask layers 106, 108, 110. Sensor performance depends, to large extent on the clean and precise definition of the sensor by the ion mill process. For example, the trackwidth of the sensor is determined by this ion milling procedure, so accurate location of the side walls is critical. In addition, accurate definition of the trackwidth depends upon having a sharp sensor edge that is as close to vertical as possible. A sloping sensor side wall results in a poorly defined trackwidth. Furthermore, a clean vertical side wall is necessary for efficient free layer biasing, since the hard bias layers will abut this side wall.
Unfortunately, the bulbous mask structure prevents clean, accurate sensor definition. For example, the bulbous mask structure results in shadowing during the image transfer process and during the sensor defining ion mill process. In addition, the bulbous mask shape result in non-uniform deposition of layers such as hard bias layers and lead layers.
Therefore, there is a strong felt need for a method for defining the track width and stripe height of a magnetoresistive sensor that overcomes the shadowing problems resulting from the bulbous mask structure described above. Such a method would preferably not result in significant added expense or process time and would preferably incorporate already implemented manufacturing processes.